Conventionally, an X-ray computed tomography (to be referred to as CT hereinafter) apparatus having a contrast medium injection monitoring mechanism monitors a CT value within an area arbitrarily designated on a displayed slice image. The X-ray computed tomography apparatus then starts a next scan (to be referred to as an actual scan) when the CT value being monitored exceeds a predetermined threshold. This can execute the actual scan in synchronism with an inflow of a contrast medium into the designated area.
However, a technique for the contrast medium injection monitoring mechanism imposes the following problems. That is, since it takes time to acquire projection data by one rotation of an X-ray tube around an object and it also takes time to reconstruct an image based on the acquired projection data, determination of an inflow of a contrast medium into an area is delayed. In addition, determination of an inflow of a contrast medium in the contrast medium injection monitoring mechanism is readily influenced by noise and artifacts in the reconstructed image.